When Shamai K. Leibowitz, an F.B.I. translator, was sentenced to 20 months in prison last year for leaking classified information to a blogger, prosecutors revealed little about the case. They identified the blogger in court papers only as “Recipient A.” After Mr. Leibowitz pleaded guilty, even the judge said he did not know exactly what Mr. Leibowitz had disclosed.

“All I know is that it’s a serious case,” Judge Alexander Williams Jr., of United States District Court in Maryland, said at the sentencing in May 2010. “I don’t know what was divulged other than some documents, and how it compromised things, I have no idea.”

Now the reason for the extraordinary secrecy surrounding the Obama administration’s first prosecution for leaking information to the news media seems clear: Mr. Leibowitz, a contract Hebrew translator, passed on secret transcripts of conversations caught on F.B.I. wiretaps of the Israeli Embassy in Washington. Those overheard by the eavesdroppers included American supporters of Israel and at least one member of Congress, according to the blogger, Richard Silverstein.

In his first interview about the case, Mr. Silverstein offered a rare glimpse of American spying on a close ally.

He said he had burned the secret documents in his Seattle backyard after Mr. Leibowitz came under investigation in mid-2009, but he recalled that there were about 200 pages of verbatim records of telephone calls and what seemed to be embassy conversations. He said that in one transcript, Israeli officials discussed their worry that their exchanges might be monitored.

Mr. Leibowitz, who declined to comment for this article, released the documents because of concerns about Israel’s aggressive efforts to influence Congress and public opinion, and fears that Israel might strike nuclear facilities in Iran, a move he saw as potentially disastrous, according to Mr. Silverstein.

While the American government routinely eavesdrops on some embassies inside the United States, intelligence collection against allies is always politically delicate, especially one as close as Israel.

The Federal Bureau of Investigation listens in on foreign embassies and officials in the United States chiefly to track foreign spies, though any intelligence it obtains on other matters is passed on to the C.I.A. and other agencies. The intercepts are carried out by the F.B.I.’s Operational Technology Division, based in Quantico, Va., according to Matthew M. Aid, an intelligence writer who describes the bureau’s monitoring in a book, “Intel Wars,” scheduled for publication in January. Translators like Mr. Leibowitz work at an F.B.I. office in Calverton, Md.

Former counterintelligence officials describe Israeli intelligence operations in the United States as quite extensive, ranking just below those of China and Russia, and F.B.I. counterintelligence agents have long kept an eye on Israeli spying.

For most eavesdropping on embassies in Washington, federal law requires the F.B.I. to obtain an order from the Foreign Intelligence Surveillance Court, which meets in secret at the Justice Department. If an American visiting or calling an embassy turns up on a recording, the F.B.I. is required by law to remove the American’s name from intelligence reports, substituting the words “U.S. person.” But raw transcripts would not necessarily have undergone such editing, called “minimization.”

Mr. Silverstein’s account could not be fully corroborated, but it fits the publicly known facts about the case. Spokesmen for the F.B.I., the Justice Department and the Israeli Embassy declined to comment on either eavesdropping on the embassy or Mr. Leibowitz’s crime. He admitted disclosing “classified information concerning the communication intelligence activities of the United States,” standard language for the interception of phone calls, e-mails and other messages by the F.B.I. and the National Security Agency, which generally focuses on international communications.

Mr. Leibowitz, now in a Federal Bureau of Prisons halfway house in Maryland, is prohibited by his plea agreement from discussing anything he learned at the F.B.I. Two lawyers who represented Mr. Leibowitz, Cary M. Feldman and Robert C. Bonsib, also would not comment.

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